In any camera that you are liable to be using, the shutter — which optically separates the film from the light — is in the camera body, and is closed except when a photo is being taken, so when you remove the lens the shutter will still protect the film.
Long ago there were exceptions - I mention them here for education and 'fun', with no real expectation that you'll actually be using a Kowa, Contaflex or Bessamatic :-).
In a small number of designs a leaf shutter is used - mounted either in the lens - or behind the lens but in front of the SLR mirror so it must be open to focus and use the viewfinder. In many cases where this is done the lens is not removable (in the usual meaning of the word). Where still used these are now invariably in medium of large format cameras, 'which you have not got'.
If you have an SLR camera with such a lens it will be a Kowa made no later than 1958 or an earlier German camera such as a Contaflex or Bessamatic. (A what?, a who? a ... ??? :-) ).
ie you won't have one. And, if you did, then it would have a "film door" behind the leaf shutter so you could remove the lens without exposing the film. If everything worked as it should :-).
Leaf shutters are what were generally used in Rangefinder cameras - when being closed as a default setting is acceptable - unlike with an SLR when the shutter blades tend to make viewing very very very difficult if closed.
Why then use them in an SLR?
The main advantages of a leaf shutter are synchronisation with flash at any speed that they will work at - typically up to 1/500 th second but 1/1000th in some designs, and lack of the distortion which occurs with high speed objects in focal plane shutters due to the shutter opening moving across the sensor/film partially open so that different parts of the moving subject are imaged on different parts of the sensor/film. Offset against this are very high complexity, higher weight, lower reliability and higher cost. Add to that the need for the complicated dance below and a film door (when film is used) and their death in SLRs was inevitable. FWIW with a DSLR a sensor door would not be strictly necessary.
Looks normal :-) (from this marvellous page
Photo copyright 2006 by Frank Mechelhoff
Interest only - here's what happen when you push the shutter button on the above Kowas:
If you press the trigger to photograph, the control circle of a KOWA
SET-R(2) shutter includes in the following steps:
- closing the shutter blades
- closing aperture to the preselected value
- mirror upward move
- film door upward move
- open the shutter again
- after preselected time, close the shutter again
- closing the film door again
- mirror downward move
- opening aperture to full value again
- opening the shutter again
Repair Notices made by Rick Oleson
Kowa with front removed - wow!
Typical modernish leaf shutter: